phosphorus. In the acid process phosphorus is not eliminated at all, but when the linings are basic it is possible to add lime and make a basic slag in which phosphorus can exist as phosphate of lime or phosphate of iron. In the acid process it is not feasible to add lime, because the lining of the converter would be eaten away and the slag could not remain basic enough to hold the phosphorus.
As already stated, the basic Bessemer process requires more heat than the acid process, because considerable lime must be added to give a basic slag, and because the lining of the vessel is eaten away much faster. It has also been explained that silicon is not allowed in the iron to any extent, because the more silicon there is present, the more lime must be added, to counteract it.
Inasmuch as silicon is the principal source of heat in the acid process, and as still more heat is required in the basic converter where silicon is not allowed, it is evident that phosphorus, which replaces silicon as a heat-producing agent, must be present in considerable quantity.- In the basic Bessemer works of Germany the iron contains about 2 per cent, of this element. If it falls much below this, the heat produced is not sufficient to give the proper temperature to the fluid metal at the end of the blow. In English practice it is considered necessary to have a higher proportion.
Thus it happens that the Bessemer process is applicable to only two kinds of ores:
(1) Those containing only a trace of phosphorus, giving an iron suitable for the acid process.
(2) Those containing a high percentage giving an iron containing 2 per cent, of phosphorus, suitable for the basic process.
There are many deposits of ore in different parts o<f the world which are intermediate between these classes, and which give a pig-iron ranging from one-tenth of one per cent, up to one and one-half per cent. These irons are not suitable for either form of the Bessemer process, although it often happens that an iron which contains too little phosphorus for the basic vessel can be used in admixture with an iron that contains a surplus. When this is impracticable, such irons can be used for steel'only in the basic open-hearth furnace.
When the air is blown through the melted iron in a basic converter the silicon is first oxidized, and the carbon next. Thus far the operation is the same in both the acid and the basic vessel.